The present invention relates to a servosystem between a master actuator and a slave actuator. Each of these actuators transmits to a control device of the other actuator a position and/or speed signal, whereby this takes place with a certain transmission delay. Although such a system can be applied to the manual control of the remote flying of a missile or to any control requiring the proportioning of the force exerted and its return to the hand of the operator, it is more particulalry suitable for the control of each of the movements of a manipulator.
So-called "force return master-slave manipulators" generally comprise a master arm, a slave arm identical or similar to the master arm and a servosystem connecting the two arms. The effect of this servosystem is that the slave arm can carry out identical or homothetic movements to those performed by the master arm and conversely the master arm can be made to follow in an identical or homothetic manner all the displacements of the slave arm. When one of the two arms encounters resistance during its displacement, the other arm encounters an equal or proportional resistance to the first-mentioned arm. Such a force return servosystem gives the control of the manipulator a completely spontaneous character and gives the user the impression of directly holding the object in his hand.
The manipulators used generally have at least six degrees of freedom, i.e. the master arm and the slave arm able to carry out at least six independent movements. Each movement of the master arm and each corresponding movement of the slave arm are carried out by independent actuators, which are interconnected in pairs of independent servosystems, the geometry of both the master arm and the slave arm recombining the different movements when a complex movement is formed.
The actuators of the force return master-slave manipulators can be electrical or hydraulic. In the most frequently encountered case of electrical actuators, which are then geared motors, importance is attached to the reduction gear being reversible and having only negligible friction, so that a master actuator is electrically connected to its corresponding slave actuator. Hydraulic actuators have a much more limited application and usually involve the manipulation of loads which are many times the force received or exerted by the operator.
No matter what type of actuator is used in the force return master-slave manipulators, two different servocontrol methods can be used.
The first of these methods, called "position-position servocontrol" consists of making the position of the slave arm follow the master arm and simultaneously making the position of the master arm follow the slave arm. This method has the obvious advantage of leading to a perfect symmetry of the servosystem. The information relating to the displacement speeds of the different parts of the master and slave arms, which are also necessary for the stability of the servosystem, can be locally processed with respect to the master or slave actuators or can be transmitted between the master and slave actuators at the same time as the arm position data. This method generally requires the reversibility of the actuators, because the device does not normally have the force transducer.
The second servocontrol method, which is called "position-force servocontrol" or "force-position servocontrol", consists of making the master arm exert the same forces as the slave arm, whilst making the slave arm occupy the same position as the master arm or vice versa. This method is obviously characterized by an asymmetry of the servosystem. In particular, the transmission carries the position information in one direction and the force information in the other direction.
The differences between the servosystems used on the force return master-slave manipulators lead to different reactions between systems, when a varyingly long delay is introduced in one of the transmission lines connecting the master arm to the slave arm in both directions.
Thus, on introducing a delay longer than 100 milliseconds into a position-position servosystem, the latter has an instability which it is possible to correct without significantly reducing the static gain of the servo control. Conversely, on wishing to carry out a displacement, the effect of the delay is to introduce a viscous friction proportional to the said delay, as well as to the displacement speed. When this delay reaches 1/2 or 1 second, the viscous friction is well above the normal displacement forces and is not really acceptable. Position-force servosystems do not have this effect, but instead have a more difficultly compensatable instability. Thus, research carried out on this type of servocontrol has revealed that it was necessary to considerably reduce the gains in order to provide a relatively stable device, when a delay of 500 to 1000 milliseconds is introduced into the transmission of the signals. Under these conditions, the force return control mode becomes virtually unusable.
If it is found that the existence of a delay in the transmission of the servosystem it is relatively frequent, it is clear that these disadvantages of the known servosystems are particularly prejudicial and it is desirable to eliminate them. This is in particular the case in servosystems using a sample of the transmitted signals. Thus, this sampling introduces a delay equal to the duration of the sampling in the transmission of the signals. This is also the case during a transmission of data over a long distance and/or in a liquid medium and particularly between a satellite and an earth station or between a submarine and a surface vessel. The intrinsic delay due to the transmission delay can then reach values between 500 and 1000 milliseconds.